Irreversible consumption of carbon sources result in depletion of fossil fuels reserves and global warming by carbon dioxide emission. This issue has prompted researchers to explore non-conventional resources, including non-food biomass, for large scale production of chemicals and fuels, particularly after the U.S. Department of Energy published a list of “ten bio-based chemicals” of top priority. Among these top priority chemicals, 5-hydroxymethylfurfural (HMF) and furfural (Ff) have received significant attention as platform chemicals for producing a broad range of chemicals and liquid transportation fuels. Despite the versatile applications of furfurals, rapid progress in developing efficient catalytic processes for conversion of carbohydrates and biomass has been witnessed over the past few years, but sustainable and economically viable routes for their production in scalable quantities has been slow to develop further.
Production of liquid fuels and chemicals from lignocellulosic plant matter (hereinafter referred to as biomass) is another integral solution to the energy grand challenge because it offers an alternative to petroleum based feedstocks. Further, “cellulosic” ethanol made from corn stalks and other agricultural feedstocks are heading towards commercialization. “Cellulosic” ethanol and other conversion methods do not make full use of the carbohydrate components of biomass (ca. 60% by weight), due to carbon loss to CO2 during fermentation of free sugars. It is noteworthy to mention as well that some modern processes also do not utilize the hemicellulose component to make ethanol due to limitations of the yeast. Regarding cellulosic ethanol and other conversion methods, they either neglect or underutilize the lignin component, a significant portion of wood biomass (15-25 wt %). Currently, most lignin is burned to produce electricity in the pulping industry and in biorefineries. As a complex biopolymer, lignin lends structural integrity to plants. Since it is composed of ether linked phenylpropanolic units, lignin contains less oxygen per carbon atom than carbohydrates (cellulose and hemicellulose) and hence, comprises ca. 40% of the energy available in biomass prior to conversion or upgrading.
Referring to FIG. 1, lignin is made by radical polymerization of three monomers (G, S, and H) to give various linkage types. The most ubiquitous linkage is the β-O-4. G, S, and H incorporation into the lignin biopolymers varies depending on the plant species and the availability of different monomers can be manipulated genetically. Moreover, this availability ultimately affects the overall lignin polymer composition in the plant. Furthermore, lignin is the only large volume renewable feedstock composed of aromatics, making it an attractive source for high value aromatic compounds, which comprise four of the top twenty chemicals in the U.S. It is noteworthy that this has been considered an attractive fuel due to the high octane.
Despite extensive research, conversion of lignin to discrete aromatic compounds remains a significant challenge. The only notable commercial process is the production of vanillin from ligno-sulfonates at a mere maximum yield of 7.5% by mass. There have been more notable developments in producing aromatics from nonaromatic biomass sources, such as bio-based styrene from butadiene produced from bio-ethanol or bio-butanol. Even though new catalysts have been reported for the cleavage of ether C—O bonds and hydrodeoxygenation (HDO) of lignin model compounds, only limited successes have been reported with lignin or biomass feedstocks. Heterogeneous Ni catalysts have been used recently with lignosulfonates to give a mixture of phenolic compounds and dimeric lignin fragments with removal of the sulfur as H2S. Ford et al. have reported a catalytic method in supercritical methanol at 300-320° C. and 160-220 bar of H2 that convert the lignified components of biomass to hydrogenated cyclic alcohols. Current methods for extraction of lignin into what is commonly known as “organosolv” lignin produce complex mixtures containing hundreds of phenolic products, none of which occur in large yields. However, universal conversion of these mixtures to valuable aromatic chemicals in a single stream product is difficult.
Aside from seeking conversion processes which produce a narrower stream of products, another approach that could be proposed is related to control of the lignin production pathway in plants. Through regulation of genes along this pathway, the base composition of lignin can be made more homogenous or even changed to contain non-native types of phenolic moieties. These techniques of biologically tailoring the biomass have great potential to create lignin that is not only easier to extract from plants in a selective manner but could also contain products that are unattainable from wild type plants.
Another widely studied approach of biomass conversion is gasification, where biomass is thermally decomposed in the presence of steam and oxygen to smaller compounds, such as carbon monoxide (CO) and hydrogen (H2), at temperatures near 800° C.-1000° C. The H2 and CO produced are subsequently recombined at lower temperatures of 250° C.-350° C. over a catalyst. This method, however, does not exploit the existing structure of the starting biomass and suffers from low overall process energy efficiency (which is defined as the ratio of energy in the products to the energy of the starting biomass and any other energy input).
Yet another approach for converting biomass to high energy density fuels and fine chemicals is liquid-phase upgrading processes. However, many of these processes do not convert the lignin fraction of biomass, thereby suppressing carbon recovery. Additional challenges to liquid-phase upgrading include finding a solvent that does not decompose at process pressures and temperatures.
Another approach for converting biomass to biofuels and other chemical commodities is based on biomass fast-pyrolysis, where biomass is thermally decomposed at 400° C.-600° C. in the presence of an inert gas to intermediate carbon chain length compounds (between 6-12 carbon atoms), which are condensed and collected as a liquid, referred to as bio-oil. This bio-oil is subsequently reacted in the liquid or vapor phase in the presence of H2 and catalytically active materials at 400° C.-600° C. to be converted to hydrocarbons. Due to the reactive nature of the oxygen containing compounds formed during fast-pyrolysis, the bio-oil tends to be unstable, acidic, and is of relatively low energy density. Therefore, re-heating of the condensed bio-oil can lead to condensation reactions that degrade the product and inhibit effective upgrading to deoxygenated hydrocarbon products.
Therefore, there is an unmet need for an energetically efficient, high yield process and catalyst that converts biomass to biofuels and other high value commodity chemicals.